It is well known that a catalyst of an exhaust gas purification system has a certain temperature region in which its exhaust gas purification performance is exhibited effectively. For example, in a case where the catalyst cannot exhibit sufficient performance because its temperature is less than a temperature at which the catalyst is active, the catalyst temperature is increased to a temperature at which the catalyst is active by placing the exhaust gas purification system in a position at the most upstream side of an exhaust passage, where the temperature of the exhaust gas is high; heating the exhaust gas purification system by means of an electric heater; or heating the exhaust gas purification system with a combustion gas generated in a combustor heater. On the other hand, when the catalyst temperature of the exhaust gas purification system increases beyond a catalyst degradation temperature, since the catalyst is degraded and the exhaust gas purification performance is lowered, the air-fuel ratio of the internal combustion engine is made richer than the theoretical air-fuel ratio, and the catalyst is cooled by utilizing the heat of vaporization of unburned fuel, thereby maintaining the catalyst temperature at less than the catalyst degradation temperature.
Furthermore, Japanese Patent Application Laid-open No. 60-93110 discloses an internal combustion engine having an exhaust gas purification system provided in an exhaust passage, a heat exchanger being disposed in each of the exhaust passage on the upstream side of the exhaust gas purification system and the exhaust passage on the downstream side thereof, thereby achieving a balance between the temperature performance of the exhaust gas purification system and the waste heat recovery performance of the heat exchangers.
Placing the exhaust gas purification system in the position at the most upstream side of the exhaust passage in order to maintain the catalyst temperature at the temperature at which the catalyst is active causes the problem that the exhaust gas purification system interferes with an internal combustion engine body or its auxiliary equipment, and the layout is difficult. Providing an electric heater or a combustor heater requires extra thermal energy, thus causing the problem that the overall energy consumption of the system increases. On the other hand, making the gas mixture rich and cooling the catalyst by means of the heat of vaporization of excess fuel in order to maintain the catalyst temperature at less than the catalyst degradation temperature causes the problem of high fuel consumption.
Moreover, in the arrangement disclosed in Japanese Patent Application Laid-open No. 60-93110, it is inherently difficult to control the catalyst temperature automatically. Since the heat exchanger provided in the exhaust passage on the upstream side of the catalyst acts as extra thermal capacity, the thermal energy of the exhaust gas is absorbed by the heat exchanger when starting the internal combustion engine from cold, and the temperature of the catalyst provided on the downstream side of the heat exchanger cannot be increased rapidly to the temperature at which the catalyst is active. On the other hand, when the catalyst temperature exceeds the catalyst degradation temperature, since the heat exchanger on the upstream side of the catalyst carries out heat exchange between the exhaust gas and the working medium to decrease the temperature of the exhaust gas, and this cooled exhaust gas cools the catalyst to a temperature below the catalyst degradation temperature, there is the problem that the temperature responsiveness of the catalyst is poor and rapid temperature control is difficult.
Furthermore, although the catalyst generates heat in an exhaust gas purification reaction, in the arrangement disclosed in Japanese Patent Application Laid-open No. 60-93110 the heat generated by the catalyst in the exhaust gas purification reaction is not utilized effectively in the heat exchanger.